The field of the present invention is generally related to computerized tomography (CT) and, particularly, to a differentially driven transmission line for high data rate communication in a CT system.
CT systems typically employ a rotating frame or gantry to obtain multiple x-ray images, or views, at different rotational angles. Each set of images is referred to in the art as a "slice". A patient or inanimate object is generally positioned in a central opening of the rotating frame on a table which is movable axially, thus enabling respective slices to be obtained at multiple axial positions as well. Each of the slices obtained is then processed in a computer according to predetermined algorithms to produce enhanced images for diagnostic or inspection purposes.
The rotating frame includes an x-ray source, a detector array and electronics necessary to generate image data for each view. A set of stationary electronics is employed for processing raw image data into the enhanced form. Thus, it is necessary to provide for communication of the image data between the rotating frame and a stationary frame of the CT system.
The data rate for communication between the stationary and rotating frames is an important factor because it is desirable to obtain the desired views as fast as possible to reduce patient discomfort and/or to maximize equipment utilization. In current CT systems, a single view typically comprises about 800 detector channels with a 16 bit representation for each individual detector channel output (i.e., 12.8 Kbits per view) and is typically repeated 1,000 times per second, yielding a net data rate requirement of approximately 13 Megabits per second (Mbit/sec) for image data alone. Future CT systems capable of simultaneously constructing multiple image slices by employing four, eight, or sixteen times as many detector channels will increase the data rate requirement to beyond 150 Mbit/sec for image data alone.
Prior CT systems have employed brushes and slip rings for electrically linking the rotating frame to the stationary frame. However, in general, CT systems utilizing brushes and slip rings for communications have generally suffered from significant limitations in the data rates which can be achieved. This is due to the substantial time required to propagate the signals around the circular slip rings. At the desired data rates, the electrical path length around the rings is an appreciable fraction of a bit period, so that electromagnetic waves propagating around the rings in opposite directions may arrive at a reception point at substantially different times in a bit period, causing garbled reception.
U.S. Pat. No. 5,208,581 issued to A. K. Collins, assigned to the assignee of the present invention and herein incorporated by reference, is another type of gantry in which brushes and slip tings are employed for communication. Although the design of Collins provides relatively high speed communication between the stationary and rotating frames, the fact remains that the use of contacting brushes and rings inherently carries certain disadvantages. For example, the mechanical contact between the brushes and rings causes wear out which requires such brushes and tings to be periodically replaced in order to maintain reliable communication. Furthermore, the slip-ring design of Collins does not support the higher data rates needed for multiple-slice CT systems.
Other CT systems have employed an optical data link for communication between the stationary and rotating frames. Although an optical data link design avoids typical drawbacks of slip rings and brushes, such optical design requires optics which must be fabricated under tight specifications and which in operation require substantial spatial alignment in order to achieve reliable optical coupling along the relatively long circumference of the rotating frame. This leads to high costs and, thus, it is desirable to provide in a CT system an improved communication link which at a low cost provides reliable high data ram communication between the stationary and rotating frames of the CT system.
It is further desirable to provide a communication link between the stationary frame and the rotating frame which is robust with respect to electromagnetic radiation interference such as is typically produced in a hospital environment by cellular telephones, defibrillating devices, surgical saws and even electrical noise produced by any given CT system. Furthermore, it is also desirable to reduce the level of electromagnetic energy which is radiated from such communication link in order to comply with governmental regulations such as regulations imposed by the Federal Communications Commission and/or foreign governments. As disclosed in U.S. patent application Ser. No. 08/307,120, a transmission line and a coupler or probe provide means for implementing such high data rate communication link. As further disclosed in U.S. patent application Ser. No. 08/307,130, a U-shaped structure is effectively employed for substantially reducing electromagnetic radiation from the transmission line while providing a passage which allows for the coupler to readily access the transmission line. Although such U-shaped structure effectively shields the transmission line, it is desirable to reduce electromagnetic radiation which escapes or leaks around the coupler. It is also desirable to reduce sensitivity of the coupler to eternally produced electromagnetic energy which can interfere with coupler operation.